1. Field of the Invention
This invention relates to a thermally responsive switch comprising a thermally responsive element and a switching element enclosed in a hermetically sealed housing, and more particularly to such a thermally responsive switch sensing the temperature of a heated portion of a compressor circulating a refrigerant to a heat exchange system, engine transmission of an automobile or the like, for protecting these equipments against the damage due to the heating.
2. Description of the Prior Art
The present invention provides an improvement of the invention assigned U.S. Pat. No. 5,121,095. Referring to FIG. 6, this patented invention provides a thermally responsive switch comprising a housing 101 with an opening. A header plate 102 is secured to the housing 101 to close its opening and has two apertures formed therethrough. First and second electrically conductive terminal pins 103A and 103B extend through the apertures of the header plate 102 and secured in them by a filler such as glass, respectively. A fixed contact member 104A and a movable contact member 104B both serving as a switching element 108 are welded to ends of the terminal pins 103A, 103B extending in the housing 101 respectively. A thermally responsive element 105 is disposed on an inner bottom face of the housing 101. The thermally responsive element 105 is formed from a bimetal and has the shape of a generally shallow dish. A support 106 formed of a leaf spring is disposed between the thermally responsive element 105 and a pressure strip 107 coupled to the movable contact member 104B. The thermally responsive element 105 reverses its curvature from a first state to a second state at a first predetermined temperature with a snap action and returns its curvature from the second state to the first state at a second predetermined temperature with the snap action. The reversing and returning movement of the thermally responsive element is transmitted to the movable contact member 104B through the support 106 and the pressure strip 107, thereby opening and closing the switching element 108.
In the above-described construction, the elastic support 106 applies a predetermined contact pressure between contact faces of the thermally responsive element 105 and the inner bottom face of the housing 101. Consequently, the thermal transmission between these contact faces is stabilized and accordingly, a stable thermal responsiveness can be maintained against changes in the ambient temperature.
In the above-described construction, however, the portions of the pins 103A, 103B where the respective fixed and movable contact members 104A, 104B are welded are on the sides opposed to those facing each other, as shown in FIG. 6. Consequently, one of a pair of welding electrodes is placed between the terminal pins 103A, 103B. Since a space between the pins 103A, 103B is excessively narrow in a miniature thermally responsive switch, the above-described construction results in the following disadvantages: a high pressure is applied to the welding electrodes so that a predetermined clamping pressure is maintained between a member to be welded and each terminal pin. However, since the space between the terminal pins is excessively narrow, it is difficult to form the welding electrode having a sufficient strength to withstand the clamping pressure for the welding or it is difficult to dispose the welding electrode between the terminal pins so that it withstands the clamping pressure. As a result, the clamping pressure is rendered unstable and accordingly, variations in the welding strength are increased. Furthermore, the service life of the welding electrode is shortened from the point of view of its strength, which results in its frequent maintenance and replacement. Moreover, since the range of the selecting arrangement of the welding electrode inserted between the terminal pins is restricted, an application of automatic welding is difficult and accordingly, the above-described construction lacks in the mass productivity. Furthermore, the reduction in the space between the terminal pins 103A, 103B is prevented by the arrangement of the welding electrode between them, which prevents further miniaturization of the thermally responsive switch.
Furthermore, when reversing or returning its curvature so as to be upwardly convex, the thermally responsive element 105 collides with the central portion of the support 106 such that the same is held between the pressure strip 107 and the thermally responsive element 105. As the result of a repeated impulsive load due to the collision, the central portion of the support 106 is gradually spread, which changes its spring characteristics. Consequently, since the elasticity applied to the thermally responsive element 105 by the support 106 is changed, the characteristics of the switching operation of the thermally responsive switch is changed accordingly.
The conventional thermally responsive switch further has a disadvantage that the members coupled to the terminal pins cannot be distinguished as the thermally responsive switch is seen from the outside after the necessary parts are incorporated in the housing. This disadvantage occurs, for example, when the polarity needs to be specified by using different materials for the contact portions between the movable and fixed contact members for the application of the thermally responsive switch to the control of a direct current. In this case, the internal arrangement coupled to the terminal pins cannot be distinguished in the conventional thermally responsive switch after completion of assembly of the housing as it is seen from the outside. A mark needs to be put on the header plate to specify a concave portion or the like, for example. However, when the mark is mistaken during the assembly of the parts into the housing, there is a possibility that the internal arrangement distinguished outside the housing differs from an intended internal arrangement.